Power electronic apparatuses with cooling arrangements

ABSTRACT

A power electronic device with cooling arrangement includes a housing that accommodates power electronic components. At least two adjacent axial fans are connected to the housing for inducing an airflow from outside into the housing in order to cool the power electronic components. Furthermore, the device can include a separating wall that extends outside the housing from between the at least two adjacent axial fans in order to reduce noise caused by the fans. Such a cooling arrangement can provide a power electronic device with effective cooling in compact size and also having an acceptable level of noise.

TECHNICAL FIELD

The present invention relates power electronic apparatuses, inparticular frequency converters, including cooling arrangements forcooling the power electronics.

In particular, the invention relates power electronic apparatuses theelectrical power of which exceeds 10 kW and may reach even megawatts.Therefore, effective cooling is required in order to safeguard theproper functioning of the apparatus.

For many applications, the most convenient cooling method is cooling byairflow induced by a fan or fans. The fans and cooling arrangements arevarious also within the air-cooled power electronic apparatuses, and therequirements of a particular system are dictated by the application forwhich the apparatus is designed.

BACKGROUND ART

US 2006/0158847 A1 discloses a cooling device for an electronicapparatus, such as a frequency converter. The cooling device comprises asingle radial fan.

US 2007/0258219 A1 discloses an air-cooled power electronics moduleprovided with a suitable number of adjacent fans.

DISCLOSURE OF INVENTION

It is an object of the present invention to create a power electronicapparatus having a new cooling arrangement that allows relativelyeffective cooling in compact size and with an acceptable noise-level.

The object of the invention is achieved by a power electronic apparatus,such as a frequency converter comprising a housing that accommodates thepower electronic components, and at least two adjacent axial fansconnected to the housing for inducing airflow from outside into thehousing in order to cool the power electronic components. Furthermore,the apparatus is provided with a separating wall that extends outsidethe housing from between the at least two adjacent axial fans in orderto reduce noise caused by the fans.

The present invention brings forth a power electronic apparatus having anew cooling arrangement that allows relatively effective cooling incompact size and with an acceptable noise-level.

The compact size and effective airflow are achieved by means of axialflow fans that are connected to the housing of the power electronicapparatus. Such powerful fans are typically noisy, but the noise levelis reduced by the afore-specified configuration of the apparatus withoutimpairment of the airflow.

Such fans can also be often driven without a separate AC converter,which also contributes to the compact size of the apparatus.

The inventive concept allows also several useful and advantageousembodiments, which provide further advantages.

According to an embodiment, each of the adjacent fans has an outerdiameter d and the separating wall extends outside the housing in adirection opposite to the direction of the airflow. In this oppositedirection, the separating wall extends for a length l such that thelength l is between 50 and 100% of the outer diameter d, such as between60 and 80% of the outer diameter d.

According to an embodiment, the separating wall has a height h in thedirection perpendicular to the direction of the airflow such that theheight h is between 50 and 300% of the outer diameter d, such as between100 and 200% of the outer diameter d. A more limited range for theheight h is between 120 and 170% of the outer diameter d, and aparticularly good value for many applications is 150% or about 150% ofthe outer diameter d.

According to an embodiment, the separating wall is substantiallyparallel with the direction of the airflow.

According to an embodiment, the adjacent axial fans and the separatingwall are all secured to a common face of the housing.

According to an embodiment, further noise reduction is achieved bydisposing at least one pair of said adjacent fans at an angle α withrespect to each other.

According to an embodiment, the angle α is between 6 and 50 degrees,such as between 10 and 20 degrees.

According to an embodiment, the fans are disposed such that the airflowsleaving the pair of said adjacent fans are inclined towards each other.

According to an embodiment, the axes of rotation of the adjacent fanssubstantially meet each other at a distant point such that the angle αcan be measured between the axes 8 of rotation. This is the case whereinthe axes of rotation extend along a common plane.

According to another embodiment, the axes of rotation of the adjacentfans do not meet each other but are tilted with respect to a commonreference plane. Then, the axes of rotation have tilting angles β withrespect to the reference plane, and the angle α can be measured betweenthe projections of the axes of rotation on the reference plane.

According to an embodiment, the tilting angles β are less than 5degrees.

As is apparent from the above disclosure, the present invention can beapplied in a great variety of applications requiring.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, the invention is now described with the aid of theexamples and with reference to the following drawings, in which:

FIG. 1 presents a frequency converter according to an embodiment;

FIG. 2 presents a frequency converter according to another embodiment;

FIG. 3 presents a cross section of the frequency converter of FIG. 1 inone possible construction;

FIG. 4 presents a cross section of the frequency converter of FIG. 1 ina construction according to a first embodiment;

FIG. 5 is a schematic view of a structural detail in the frequencyconverter of FIG. 4;

FIGS. 6A and 6B present cross sections of the frequency converter ofFIG. 1 in two modifications of a construction according to a secondembodiment;

FIGS. 7A and 7B are schematic views of structural details in thefrequency converters of FIGS. 6A and 6B;

FIG. 8 presents a cross section of the frequency converter of FIG. 1 ina construction according to a third embodiment;

FIG. 9 presents an embodiment of the frequency converter with threefans;

FIG. 10 presents another embodiment of the frequency converter withthree fans;

FIG. 11 presents an embodiment of the frequency converter with fourfans;

FIG. 12 presents another embodiment of the frequency converter with fourfans;

FIG. 13 presents a further embodiment of the frequency converter withfour fans; and

FIG. 14 presents a further embodiment of the frequency converter withthree fans.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 presents a housing 1 of a frequency converter. The frequencyconverter comprises power electronic components (not shown) inside thehousing 1 that dissipate heat and require cooling. The frequencyconverter of FIG. 1 is of the air-cooled type. Therefore, a front face 2of the housing 1 defines two apertures acting as air inlets 3. FIG. 1also shows two adjacent fans 4 fitted into the air inlets 3 for blowingcooling air inside the housing. The arrows in the Figure depict airmovement induced by the fans 4 when they operate. In the embodiment ofFIG. 1, the housing comprises air outlets at its back face, which islocated at the opposite end of the housing with regard to the front face2. The back face of the housing is however not visible in the Figure. Itis also possible to locate the air outlets at other faces of thehousing, if desired. For example, the air could be directed to circulateinside the housing and be lead outside the housing through air outletsat the front face 2 or the side panels of the housing.

The number of fans is not limited but can be selected according to therequirements of each frequency converter application. FIG. 2 shows onesuch modification having four adjacent fans 4 mounted into the housing1.

The power dissipation is considerable in the frequency convertersaccording to the embodiments. The electrical power of the frequencyconverter can be between 10 kW and 5 MW, for instance. Then, the thermalpower dissipated inside the housing 1 can be very considerable. In orderto effectively cool the power electronic components by means ofair-cooling, the frequency converter is provided with a sufficientnumber of air fans 4.

According to the embodiments, the fans 4 are of the axial flow type. Anaxial flow fan comprises a number of blades attached to a central hub,and in operation, the central hub is rotated around an axis of rotation,whereby the blades cause air to flow in a flow direction generallyparallel with the axis of rotation in case the airflow is not deflected.In this document, the direction of a fan refers to the direction of suchnon-deflected general flow direction and, at the same time, also to thedirection of the axis of rotation.

The fans 4 according to the embodiment are also in itself relativelypowerful in order to achieve the necessary airflow through the housing1. The power of each fan 4 can be between 10 W and 1 000 W, forinstance. In apparatuses having higher power consumption, fans 4 withthe input power of at least 50 W are typically used. Examples of suchfans include Papst 6314/2TDHHP with the input power of 67 W and JapanServo D1751S24B8ZP300 with the input power of 82 W. The level of noisecan be in the range 65 to 85 dB when such fans operate.

In an embodiment, the electrical power of the frequency converter isselected from between 10 kW and 500 kW, and the input power of the fans4 is between 60 W and 200 W.

FIG. 1 shows a compact arrangement of the frequency converter with thefans 4 and the air inlets 3 taking up a large portion of the area of thefront face of the housing 1. This facilitates in achieving a highcooling airflow relative to the size of the housing 1. A drawback isthat the two relatively high-power fans 4 create considerable noise.

FIG. 3 presents a basic construction of a frequency converter accordingto FIG. 1 without the noise reduction modifications according to theembodiments of the invention. FIG. 3 is a schematic top viewcross-section the frequency converter, showing the housing 1, powerelectronic components 5, air inlets 3, the two adjacent fans 4 and airoutlet 6 at the back face of the housing 1.

In a first test set-up, the noise was measured when using one fan 4 at atime in the basic construction of the frequency converter as shown inFIG. 3. One of the fans was Papst 6314/2TDHHP with the power of 67 W andthe other was Japan Servo D1751S24B8ZP300 with the power of 82 W. It wasfound that, when the fans were operated one at a time, the noise levelmeasured at a reference point was 74.0 dB for the Papst 6314/2TDHHP fanand 70.3 dB for the Japan Servo D1751S24B8ZP300 fan. Thus, the basicconstruction of FIG. 3 should lead to noise levels of about 3 dB higherthan the above values when operating both of the adjacent fans 4 at thesame time.

FIG. 4 presents a schematic top view cross-section of the frequencyconverter according to FIG. 1 including a noise reduction arrangementaccording to a first embodiment. In the first embodiment, the adjacentfans 4 are fitted in a tilted arrangement such that the general airflowdirections are inclined towards each other.

FIG. 5 shows more closely the tilted arrangement of FIG. 4. FIG. 5 showsthe fans 4 and a grating 7 covering the air inlets 3 shown in FIG. 4.FIG. 5 depicts also the axes 8 of rotation of the fans 8, and an angle αbetween the axes. Thus, the fans 4 are disposed at an angle α withrespect to each other. In the embodiment of FIG. 5, both of the axes 8of rotation extend along the plane of the cross-section depicted in theFIG. 5. The direction of the angle α is such that the axes 8 of rotationare inclined towards each other. Thus, in FIG. 5, the plane of thecross-section is applicable as the reference plane for defining theangle. However, there are also embodiments in which one or several ofthe fans 4 may be tilted also in the perpendicular direction andtherefore there is need to define a more general reference plane for theangles.

In general, the reference plane for the angles goes via the central hubsof the fans and follows the general non-deflected flow direction of theair leaving the fans. Such a general flow direction of the air is ofcourse somewhat imaginary in a practical apparatus set-up, as thehousing 1 and other environment factors always guide the airflow atleast to some extent. However, a skilled reader understands that thereference plane is needed in order to clearly define the angles andshould not be ascribed other technical meaning.

As is apparent from the above discussion, the fans 4 can also be fittedsuch that the axes 8 of rotation extend at an angle with respect to thereference plane, such as the plane of the cross-section in FIG. 5. Then,the axis 8 of rotation defines an angle β with respect to the referenceplane. This angle is orthogonal to the angle α and can be individuallyset for each of the fans 4. Hence, in an embodiment having two fans 4,the first and second fans 4 have respectively first and second angles β₁and β₂. In typical embodiments, the absolute values of the angles β₁ andβ₂ are substantially equal but the angles are opposite in direction. Theabsolute values of the angles β₁ and β₂ can be less than 10 degrees, forinstance. More typically, the absolute values of the angles β₁ and β₂are less than 5 degrees. In case the absolute values of the angles β₁and β₂ are less than 2 degrees, it can be considered that the embodimentsubstantially correspond to that of FIGS. 4 and 5.

In FIGS. 4 and 5, the angle α is 10 degrees. In general, the angle α canbe between 6 and 50 degrees, for instance. In case the angle is less, itdoes not sufficiently aid in reducing the noise. On the other hand,greater angles are not beneficial in view of the airflow and lead alsoto inefficient use of space in the housing 1. When taking these aspectsinto account, it is expected that, for most of the applications, a goodvalue for the angle α can be found in the range between 10 and 20degrees.

The angle of about 10 degrees was found effective in reducing noise ofthe fans without impairment in the flow of cooling air also inmeasurements made with a second test set-up. The second test set-upcorresponded otherwise to the first test set-up described above, exceptthat there was an angle α of about 10 degrees between the axes 8 of theadjacent fans 4. The angles β₁ and β₂ were negligible, i.e. less than 2degrees. The noise level of 65.8 dB was measured when operating twoJapan Servo D1751S24B8ZP300 fans simultaneously. Thus, the noise levelat the reference point was about 4.5 dB lower than when operating onlyone of the fans 4. If we take into account that the noise level shouldhave been raised when using two identical fans 4 instead of one fan, weconclude that considerable reduction of noise was achieved by means ofthe angle α.

FIG. 6A presents a schematic top view cross-section of the frequencyconverter according to FIG. 1 including a noise reduction arrangementaccording to a second embodiment. In the second embodiment, the adjacentfans 4 are fitted in a tilted arrangement such that the general airflowdirections are inclined towards each other. In addition, a separatingwall 9 is provided between the adjacent fans 4 in order to separate theairflows from the fans 4. In the modification presented in FIG. 6A, thefans 4 are fitted inside the housing 1.

FIG. 6B presents a schematic top view cross-section of the frequencyconverter otherwise similar to that of FIG. 6A but having the fans 4 arefitted outside the housing 1. As can be seen in FIGS. 6A and 6B, thismodification presented in 6B also allows the housing 1 to bemanufactured shorter because no space for the fans 4 need be reservedinside the housing 1.

FIGS. 7A and 7B show more closely the tilted arrangement and theseparating wall 9 of FIGS. 6A and 6B, respectively. With regard to theangles and inclinations, the second embodiment corresponds to the firstembodiment already described above by reference to FIGS. 4 and 5. Theadditional feature in the second embodiment is the separating wall 9secured between the two adjacent fans 4 outside the housing 1. FIGS. 7Aand 7B also show fan housings 14 into which the actual fans 4 arefitted. Each fan housing 14 has a diameter d, hereinafter referred toalso as the diameter of the fan 4, and a depth α, hereinafter referredto also as the depth of the fan 4.

The separating wall 9 extends a length l in a direction opposite to theflow direction of the air. The length l is 11 cm in the embodiments ofFIGS. 6A and 6B. The length l is as measured from the level of theclosest edges of the fans 4 or fan housings 14, if any. In case thelength is too small, the separating wall 9 does not sufficiently aid inreducing the noise. On the other hand, too long a wall would take up toomuch space and would therefore be unacceptable when aiming at a compactstructure with low noise. In the embodiments resembling those of FIGS.6A and 6B, the length l could be selected between 8 and 16 cm, forinstance, when the diameter of the fan 4 is between 16 and 18 cm. Asalready stated above, the diameter of the fan 4 refers to the outerdiameter of the fan housing 14, which is about 17 cm for both of the fantypes specified above.

More generally, the length l of the separating wall 9 could be selectedto be between 20 and 130% of the outer diameter of the fan housing 14.It is expected that, for most of the applications, a good value for thelength l can be found in the range between 30 and 60% of the outerdiameter of the fan housing 14. The outer diameter of the fan housingitself can be selected according to the needs of the application.Typical outer diameters may range between 15 and 40 cm, for instance.

An alternative way to define the length l of the separating wall 9 moregenerally is with reference to the depth a of the fan housing 14.Defined by this way, the length l of the separating wall 9 could beselected to be between 50 and 400% of the depth a of the fan housing 14.It is expected that, for most of the applications, a good value for thelength l can be found in the range between 80 and 200% of the depth ofthe fan housing 14. Typical depths may range between 4 and 15 cm, forinstance.

When designing the length of the separating wall 9, it should be notedthat the actual component forming the wall is longer than theabove-referred length l in the embodiment shown in FIGS. 6B and 7B. Thisis because the length l is measured from the edge of the fan housing 14(as shown in FIG. 7B) and not from the front panel to which theseparating wall 9 is secured in the embodiment of FIGS. 6B and 7B.

Another parameter of the separating wall 9 is its height h in thedirection perpendicular to the length l. The height h can be selected,for example, between 50 and 300% of the outer diameter of the fanhousing. It is expected that, for most of the applications, a good valuefor the height h can be found in the range between 100 and 200% of theouter diameter of the fan housing. A particularly good value for manyapplications is 150% or about 150% of the outer diameter d.

The separating wall 9 is made of a sufficiently rigid material, such asmetal or hard plastic, and it is preferable unified in the sense that itdoes not contain holes allowing airflows to interfere with each other.

The use of a separating wall 9 between the adjacent fans was found tofurther reduce the noise level with regard to the apparatus set-upaccording to the second embodiment. This was found by measurements of athird test set-up that corresponded otherwise to the first test set-updescribed above, except that there was an angle α of about 10 degreesbetween the axes 8 of the adjacent fans 4 and a separating wall 9 with alength l of 11 cm between the fans 4. The angles β₁ and β₂ werenegligible, i.e. less than 2 degrees. The noise level of 65.6 dB wasmeasured when operating two Japan Servo D1751S24B8ZP300 fanssimultaneously. When using two Papst 6314/2TDHHP fans simultaneously,the measured noise level was 68.4 dB at the same reference point. Thus,for the two simultaneously operating Papst fans, this embodiment gaveabout 5.6 dB noise reduction with regard to the measurements performedwith only of such fans turned on.

The effect of a separating wall 9 between the adjacent fans was foundalso when measuring noise in a fourth test set-up that was otherwiseaccording to the above test set-ups but relied solely on a separatingwall 9 as the means for noise reduction. The separating wall 9 was madeof plastic, and its length l was 10 cm. The angles α, β₁ and β₂ werenegligible. The noise level of 70.9 dB was measured when operating twoJapan Servo D1751S24B8ZP300 fans simultaneously. When using two Papst6314/2TDHHP fans simultaneously, the measured noise level was 74 dB atthe same reference point. Thus, it can be seen that noise reduction wasattained as the noise levels were not increased by about 3 dB, whichincrease was expected in context of the first test set-up used as thereference.

Therefore, it was shown that further noise reduction can be obtained bymeans of a separating wall. Even though the effect of the separatingwall is not as great as the effect of the angle α, it is reasonable toprovide the separating wall at least in such applications of thefrequency converter wherein the noise level is critical and the housing1 can readily accommodate the separating wall. The separating wallitself is not expensive and as it does not impair the cooling airflow,it is a reasonable and safe measure to use in such application. Thus, afurther embodiment is also shown in FIG. 8, in which the basicconstruction of the frequency converter shown in FIG. 3 is provided withthe separating wall as described above by reference to FIGS. 6 and 8.

The above-described embodiments can, of course, be modified in variousways.

For example, the number of fans 4 can be varied according to the needsof the applications. FIG. 2 shows a frequency converter having fourfans. In such a configuration, it is possible to fit the fans as pairs,for example such that the two left-side fans are inclined towards eachother as described above and the two right-side fans are also tiltedaccordingly. If desired, separating walls can be provided between eachadjacent fan. The same procedure can be followed when there is an evennumber of fans and the configuration allows treating them as pairs. Incase of odd number of fans or the fans being unpaired, it is possiblethat only some of the fans 4 are installed according to the aboveembodiments whereas the rest of the fans act alone without using theabove means for reducing noise.

It is also possible that for example three fans are all inclined towardseach other in a triangular configuration. FIG. 9 is a schematic drawingof such a triangular configuration in which three fans 4 are allinclined towards a common imaginary centre axis 10 shown in dotted line.FIG. 9 also shows one possible construction of separating walls 9 to beused in such a configuration. The separating walls 9 are formed by threeplates shown in the Figure.

FIG. 10 shows an embodiment, wherein three fans 4 are placed adjacentlyin a row. The three fans 4 can be place for example such that theleft-side fan and the right-side fan are inclined towards the fan at thecentre. An imaginary centre axis (not shown) coincides with the axis ofrotation of the fans at the centre and the fans at the left and rightsides are inclined towards the common imaginary centre axis. The anglesof inclination and the other parameters can generally follow thosedescribed above in context with the other embodiments. It is alsopossible to provide separating walls 9 between the fans as is shown inthe Figure.

FIG. 11 depicts an embodiment wherein the front panel has been dividedinto four curved front plates 13 to which a corresponding number of fans4 have been fitted. All four fans 4 can be inclined towards a commonimaginary centre axis 10 shown in dotted line.

FIG. 12 depicts an embodiment wherein the front panel has been dividedinto four straight front plates 13 fitted in angular relationship witheach other. As shown in FIG. 12, there is a fan 4 secured to each of thefour front plates 13. In this embodiment, the fan housings 14 can besecured to the front plates 13 without an angle between the fan housing14 and the front plate 13, and it is still possible to achieve theangular relationship between the fans 4 by means of the angles betweenthe front plates 13. Also in this embodiment, it is possible, eventhough in no means obligatory, to incline the all four fans 4 towards acommon imaginary centre axis 10.

FIG. 13 depicts an embodiment otherwise identical to the embodiment ofFIG. 12 but provided with separating walls 9 in front of the apparatus.

FIG. 14 depicts an embodiment wherein the front panel has been dividedinto three straight front plates 13 in succession. The fans 4 aresecured to the outer surfaces of the front plates 13 and there is noangle between the fan 4 and the associated front plate 13. Theembodiment has a separating wall 9 between first and second fans 4(counting from the left side of FIG. 14) and an angle α (as describedabove) between the second and third fans 4 (again counting from the leftside of FIG. 14). Thus, the noise is reduced by the angle α between thesecond and third fans 4 and by the separating wall 9 between the firstand second fans 4. As can be seen in FIG. 14, the first and second fans4 are disposed at an angle that is opposite in direction that the angleα used in noise reduction and therefore the separating wall 9 isprovided between these fans. However, there is no separating wall 9between the second and third fans 4 because the noise is reduced bymeans of the angle α and any further reduction by the separating wall 9is not necessary. Furthermore, a separating wall 9 between the secondand third fans 4 would extend outwardly from the front face of theapparatus and therefore increase the outer dimensions of the apparatus.

Also in the embodiments of FIGS. 11 to 14, the angles of inclination andthe other parameters can generally follow those described above incontext with the other embodiments.

As can be seen in FIGS. 11 to 13, the apparatus can comprise four fansplaced in two adjacent rows such that each of the fans 4 forms a pairwith two fans 4, one of which is in the same row and another in theadjacent row. Furthermore, each of such pairs can have the fans 4disposed at the angle α with respect to each other, in which the angle αis as specified above in context with other embodiments. Such anapparatus may of course comprise, for example, six or eight fansdisposed in a same fashion.

FIG. 13 also shows that the apparatus comprising four fans placed in twoadjacent rows as described above also has a separating wall for each ofsaid pairs of the fans.

As can be seen in FIGS. 11 to 14, the housing 1 of the apparatus cancomprise a front face 2 that has at least two adjacent front plates 13.Furthermore, each of the at least two adjacent front plates 13 has atleast one fan 4 secured thereto, and the front plates 13 are disposed atthe angle with respect to each other. Again, the angle α is as specifiedabove in context with other embodiments.

FIGS. 13 and 14 also show that the apparatus comprising the front face 2with at least two adjacent front plates 13 as described above also has aseparating wall 9 secured between said at least two adjacent frontplates 13.

The minimum distance between the adjacent fans 4 is typically between20% and 100% of the diameter d of the fan 4. Generally, the minimumdistance between the adjacent fans 4 can be designed to be less than200% of the diameter d. In case the adjacent fans have differentdiameters, the minimum distance can be calculated on the basis ofdiameter of the fan 4 having the smaller diameter.

The above description is only to exemplify the invention and is notintended to limit the scope of protection offered by the claims. Theclaims are also intended to cover the equivalents thereof and not to beconstrued literally.

The invention claimed is:
 1. A power electronic apparatus, such as afrequency converter, comprising a housing; at least one power electroniccomponent inside the housing; at least two adjacent axial fans connectedto the housing for inducing an airflow from outside into the housing inorder to cool the at least one power electronic component; and aseparating wall extending outside the housing from between the at leasttwo adjacent axial fans, the separating wall extending in a directionopposite to a direction of the airflow wherein each of the adjacent fanshas an axis of rotation and at least one pair of said adjacent fans isdisposed at an angle α with respect to each other, wherein the angle αis measured between the axes of rotation and the direction of the angleα is such that the airflows leaving the pair of said adjacent fans areinclined towards each other, and wherein the angle α is between 6 and 25degrees.
 2. The power electronic apparatus of claim 1, wherein each ofthe adjacent fans has an outer diameter d and the separating wallextends outside the housing in the direction opposite to the airflow fora length l such that the length l is between 50 and 100% of the outerdiameter d.
 3. The power electronic apparatus of claim 2, wherein thelength l is between 60 and 80% of the outer diameter d.
 4. The powerelectronic apparatus of claim 1, wherein the separating wall has aheight h in the direction perpendicular to the direction of the airflowsuch that the height h is between 50 and 300% of the outer diameter d.5. The power electronic apparatus of claim 4, wherein the height h isbetween 100 and 200% of the outer diameter d.
 6. The power electronicapparatus of claim 4, wherein the height h is between 120 and 170% ofthe outer diameter d.
 7. The power electronic apparatus of claim 1,wherein the separating wall is substantially parallel with the directionof the airflow.
 8. The power electronic apparatus of claim 1, whereinthe power of the power electronic apparatus is at least 10 kilowatts. 9.The power electronic apparatus of claim 1, wherein the power of each ofthe adjacent axial fans is at least 50 watts.
 10. The power electronicapparatus of claim 1, wherein both the adjacent axial fans and theseparating wall are secured to a front face of the housing.
 11. Thepower electronic apparatus of claim 1, wherein each of the adjacent fanshas an axis of rotation having a projection on a reference plane, thereference plane extending in the direction of the airflow and goingthrough the center points of the adjacent fans, and the axes of rotationdefining angles β with respect to the reference plane, and wherein theangle α is measured between said projections of the axes of rotation onsaid reference plane, and wherein the angles β are less than 5 degrees.12. The power electronic apparatus of claim 1, comprising four fansplaced in two adjacent rows such that for each of the fans there are twofans, one of which is in the same row and another in the adjacent row,that each forms a pair with the said fan, and the apparatus comprisesanother separating wall for each of said pairs of the fans.
 13. Thepower electronic apparatus of claim 1, wherein the housing comprises afront face having at least two adjacent front plates, each of the atleast two adjacent front plates having at least one fan secured thereto,and wherein the separating wall is secured between said at least twoadjacent front plates.
 14. A frequency converter, comprising: a housing;at least one power electronic component inside the housing; at least twoadjacent axial fans connected to the housing for inducing an airflowfrom outside into the housing in order to cool the at least one powerelectronic component; and a separating wall extending outside thehousing from between the at least two adjacent axial fans, theseparating wall extending in a direction opposite to a direction of theairflow, wherein the angle α is measured between the axes of rotationand the direction of the angle α is such that the airflows leaving thepair of said adjacent fans are inclined towards each other, and whereinthe angle α is between 6 and 25 degrees.
 15. The frequency converter ofclaim 14, wherein each of the adjacent fans has an outer diameter d andthe separating wall extends outside the housing in the directionopposite to the airflow for a length l such that the length l is between50 and 100% of the outer diameter d.
 16. The frequency converter ofclaim 14, wherein the separating wall has a height h in the directionperpendicular to the direction of the airflow such that the height h isbetween 50 and 300% of the outer diameter d.
 17. The frequency converterof claim 14, wherein the separating wall is substantially parallel withthe direction of the airflow.
 18. The frequency converter of claim 14,comprising four fans placed in two adjacent rows such that for each ofthe fans there are two fans, one of which is in the same row and anotherin the adjacent row, that each forms a pair with the said fan, and theapparatus comprises another separating wall for each of said pairs ofthe fans.
 19. The frequency converter of claim 14, wherein the housingcomprises a front face having at least two adjacent front plates, eachof the at least two adjacent front plates having at least one fansecured thereto, and wherein the separating wall is secured between saidat least two adjacent front plates.